• Energy Research

Abstract There is a growing need for drop-in biofuels for gas turbines for enhanced energy security and sustainability. Several fuels are currently being developed and tested to reduce dependency on fossil fuels while maintaining performance, particularly in the aviation industry. The transition from traditional fossil fuels to sustainable biofuels is much desired for reducing the rapidly rising CO2 levels in the environment. This requires biofuels to be drop-in ready, where there are no adverse effects on performance and emissions upon replacement. In this study, the performance and emissions of four different aviation drop-in biofuels were evaluated. They include UOP HEFA-SPK, Gevo ATJ, Amyris Farnesane, and SB-JP-8. These aviation biofuels are currently being produced and tested to be ready for full or partial drop-in fuels as the replacement of traditional jet fuels. The characteristic performance of each fuel from the prevaporized liquid fuels was performed in a high-intensity (20 MW/m3-atm) reverse flow combustor. The NO emissions showed near unity ppm levels for each of the fuels examined with a minimum at an equivalence ratio of ∼0.6, while CO levels were in the range of 1000–1300 ppm depending on the fuel at an equivalence ratio between 0.75 and 0.8. For an equivalence ratio range between 0.4 and 0.6, NO and CO emissions remained very low (between 1–2 ppm NO and 2400–2900 ppm CO) depending on the fuel. The examined biofuels did not show any instability over a wide range of equivalence ratios from lean to near stoichiometric condition. These results provide promising results on the behavior of these drop-in aviation biofuels for use in high-intensity gas turbine combustors providing stability and cleaner performance without any modification to the combustor design.

Abstract High intensity distributed combustion assists to provide substantial performance improvement for gas turbine applications for our quest to simultaneously seek improved pattern factor, ultra-low emission of NOx and CO, low noise, enhanced stability, fuel flexibility and higher efficiency. In such combustion method, controlled mixing between the injected air, fuel and hot reactive gases from within the combustor prior to mixture ignition must occur to achieve distributed reactions in the entire combustion chamber. Near zero emission of NO and CO has been achieved using methane as the fuel under distributed combustion conditions at high heat (energy) release intensity of 22–36 MW/m3 atm. The conditions to form distributed reaction in the combustor are further investigated through variation of air injection velocity with the output parameters focused on pollutants emission and combustor performance. The isothermal flowfield is examined using particle image velocimetry (PIV) to determine key features associated with the flowfield and its effects on pollutants emission and stability. The results showed higher entrainment and turbulence at increased injection velocity. Increase in injection velocity decreased NO emissions by some 20–48% with minimal impact on CO emission under premixed fuel–air condition. Less than 4 ppm of NO was achieved at an injection velocity of 46 m/s at an equivalence ratio of 0.7 and heat (energy) release intensity of 31.5 MW/m3 atm using normal temperature air. High injection velocity at the same operating condition decreased NO emission by some 20% to 3.2 ppm. Higher injection velocity under preheated inlet air condition further decreased NO to 2 ppm (48% reduction) at an equivalence ratio of 0.5. The results under non-premixed combustion conditions showed similar behavior. The reduction of NO with single injection parameter is attributed to improved distributed reaction condition from direct entrainment and rapid mixing of reactive species present in the combustion zone under high intensity combustion conditions.

Abstract Colorless Distributed Combustion is a combustion technique to reduce pollutants emission, reduce noise, enhance flame stability, promote a uniform thermal field distribution in the flame, and mitigate combustion instability. To achieve these conditions, hot reactive gases must be entrained into the oxidizer to reduce the oxygen concentration, which slows down the reaction rate to broaden and homogenize the reaction front. This paper focuses on the development of a distributed combustion index that will predict transition to favorable distributed combustion conditions over a range of conditions. Distributed conditions were achieved by adding either N2 or CO2 dilution to the oxidizer stream to simulate hot gas entrainment. The index developed here is for either N2 or CO2 dilution and focuses on the effects of heat release intensity, equivalence ratio, and mixture preheat temperature. The results show heat release intensities in the range of 5.72–9.53 MW/m3-atm to have minimal effect on the oxygen concentration corresponding to transition to distributed combustion. In contrast, equivalence ratio and preheat temperature provided strong effects on this transition. Decrease in equivalence ratio from 0.9 to 0.6 required increased oxygen concentration for transition to distributed conditions by some 4% with N2 as the diluent and only 3% with CO2 as the diluent. Increase in mixture preheat temperature from 300 to 700 K decreased oxygen concentration transition requirements by some 3% with N2 and 2% for CO2. Emission levels obtained showed ultra-low NO and CO under favorable distributed combustion condition. The distributed combustion index development presented here is aimed to help guide in the design and development of novel next generation of advanced colorless distributed combustors with much reduced further experimentation.

Abstract In this investigation the role of hydrogen addition in a reverse flow configuration, consisting of both non-premixed and premixed combustion modes, have been examined for the CDC flames. In the non-premixed configuration the air injection port is positioned at combustor exit end while the fuel injection port is positioned on the side so that the fuel is injected in cross-flow with respect to air injection. The thermal intensity of the flames investigated is 85 MW/m 3 atm to simulate high thermal intensity gas turbine combustion conditions. The results are presented on the global flame signatures, exhaust emissions, and radical emissions using experiments and flowfield using numerical simulations. Ultra low NO x emissions are found for both the premixed and non-premixed combustion modes. Addition of hydrogen to methane fuel resulted in only a slight increase of NO emission, significant decrease of CO emission and extended the lean operational limit of the combustor.

Abstract Combined use of plastic and biomass wastes offers promising pathway for simultaneous energy production and waste disposal. In this article, the co-pyrolysis of pinewood and polycarbonate (PC) was performed in a fixed bed reactor to quantify their synergistic interaction on the product output and determine the effect of char intermediates on the synergistic effects. The extent of synergistic effects was obtained via a direct comparison of results from co-pyrolysis of pinewood–polycarbonate mixture with the weighted average values from pyrolysis of individual components. The observed synergistic effects were further examined from the influence of char intermediates using tailored feedstock configurations to gain more insights into the synergistic mechanism. The results showed co-pyrolysis resulted in enhancement by 33% in H2, 26% in CO, and 19% in total syngas yields compared to their weighted values from individual pyrolysis. Co-pyrolysis also exhibited superiority in energy recovery with the overall energy efficiency promoted from 42.9% to 48.6%. Deconvolution of synergistic effects revealed that pinewood char catalytically enhanced PC degradation, while the effect of PC char on pinewood pyrolysis was minimal. This article provides results on deconvolved understanding of synergistic effects in co-pyrolysis of lignocellulosic biomass and PC wastes, which is very helpful in designing clean and efficient energy recovery systems from these waste resources.

Abstract Lean blowoff in distributed combustion was investigated at moderate heat release intensities of 5.72, 7.63, and 9.53 MW/m3-atm to characterize the blowoff phenomenon. Distributed combustion conditions were established from a conventional swirl flame at an equivalence ratio of 0.9 using carbon dioxide as the diluent to the inlet airstream. A gradual increase in the air flowrate provided a reduction of equivalence ratio that eventually resulted in the lean blowoff limit. Blowoff occurred at relatively higher equivalence ratios for higher heat release intensities, which was attributed to higher inlet turbulence leading to the early introduction of flame instabilities and blowoff. High-speed chemiluminescence imaging (at 500 frames/second) performed near blowoff moments demonstrated the transition of distributed reaction zone to a near V-shape zone due to quenching of flame surface along the sides. A closer examination of the reduction in equivalence ratio in small steps near the global blowoff showed the presence of a very thin thread-like rotating reaction zone. The observations of blowoff were further supported by the analysis of chemiluminescence signals in each case. The effect of inlet air preheats on blowoff was also investigated. Air preheats broadened the lean blowoff to a lower equivalence ratio which was attributed to enhanced flame speed, providing additional flame stability and reduction of flowfield instabilities. The laminar flame speeds obtained at each preheats case using Chemkin-Pro© simulation with GRI-Mech 3.0 reaction mechanisms supported such a hypothesis of gradually enhanced flame speed, providing additional flame stability.

In recent years, turbine inlet temperatures in gas turbine systems have continued to rise in order to enhance efficiency and performance. However, higher temperatures results in increased levels of NO x emission and this in turn seriously affect the environment. Development of prediction methods for NO x emissions is important, since this allows one to evaluate the environmental contributions of gas turbine systems. The gas turbine combustor geometries are very complex so that the flow pattern, chemical and thermal field inside the combustor is very complex. Numerical methods for predicting the performance of the combustor and NO x formation and emission levels are attractive as they provide inexpensive solutions to the complex geometry problem without physically building the hardware. This paper provides numerical simulations for predicting the NO x emissions from gas turbine systems using: ( i ) chemical equilibrium and simplified kinetic reaction approaches, and (ii) flamelet and reaction progress variable approaches.

Abstract The fundamentals and thermodynamic analysis of high-temperature air combustion (HiTAC) technology is presented. The HiTAC is characterized by high temperature of combustion air having low oxygen concentration. This study provides a theoretical analysis of HiTAC process from the thermodynamic point of view. The results demonstrate the possibilities of reducing thermodynamic irreversibility of combustion by considering an oxygen-deficient combustion process that utilizes both gas and heat recirculations. HiTAC conditions reduce irreversibility. Furthermore, combustion with the use of oxygen (in place of air) is also analyzed. The results showed that a system, which utilizes oxygen as an oxidizer, results in higher first and second law efficiencies as compared to the case with air as the oxidizer. The entropy generation for an adiabatic combustion process is reduced by more than 60% due to the effect of either preheating or oxygen enrichment. This study is aimed at providing technical guidance to further improve efficiency of a combustion process, which shows very small temperature increases due to mild chemical reactions.

Abstract The final carbon conversion rate is of critical importance in the efficiency of gasifiers. Therefore, comprehensive modeling of char particle conversion is of primary interest for designing new gasifiers. This work presents a novel intrinsic-based submodel for the gasification of a char particle moving in a hot flue gas environment considering CO2 and H2O as inlet species. The first part of the manuscript describes the model and its derivation. Validations against experiments carried out in this work for German lignite char are reported in the second part. The comparison between submodel predictions and experimental data shows good agreement. The importance of char porosity change during gasification is demonstrated. The third part presents the results of CFD simulations using the new submodel and a surface-based submodel for a generic endothermic gasifier. The focus of CFD simulations is to demonstrate the crucial role of intrinsic based heterogeneous reactions in the adequate prediction of carbon conversion rates.

Abstract New innovative advanced combustion design methodology for gas turbine applications is presented that is focused on the quest towards zero emissions. The new design methodology is called colorless distributed combustion (CDC) and is significantly different from the currently used methodology. In this paper forward flow modes of CDC have been investigated for application to gas turbine combustors. The CDC provides significant improvement in pattern factor, reduced NOx emission and uniform thermal field in the entire combustion zone for it to be called as an isothermal reactor. Basic requirement for CDC is carefully tailored mixture preparation through good mixing between the combustion air and product gases prior to rapid mixing with fuel so that the reactants are at much higher temperature to result in hot and diluted oxidant stream at temperatures that are high enough to autoignite the fuel and oxidant mixture. With desirable conditions one can achieve spontaneous ignition of the fuel with distributed combustion reactions. Distributed reactions can also be achieved in premixed mode of operation with sufficient entrainment of burned gases and faster turbulent mixing between the reactants. In the present investigation forward flow modes consisting of two non-premixed combustion modes and one premixed combustion mode have been examined that provide potential for CDC. In all the configurations the air injection port is positioned at the opposite side of the combustor exit, whereas the location of fuel injection ports is changed to give different configurations. Two combustion geometries resulting in thermal intensity of 5 MW/m3-atm and 28 MW/m3-atm are investigated. Increase in thermal intensity (lower combustion volume) presents many challenges, such as, lower residence time, lower recirculation of gases and effect of confinement on jet characteristics. The results are presented on the global flame signatures, exhaust emissions, and radical emissions using experiments and flowfield using numerical simulations. Ultra-low NOx emissions are found for both the premixed and non-premixed combustion modes at the two thermal intensities investigated here. Almost colorless flames (no visible flame signatures) have been observed for the premixed combustion mode. The reaction zone is observed to be significantly different in the two non-premixed modes. Higher thermal intensity case resulted in lower recirculation of gases within the combustion chamber and higher CO levels, possibly due to lower associated residence time. The characteristics at the two thermal intensity combustors investigated here were found to be similar.